Six people were killed and eight others injured when a rubbish truck ploughed into Christmas shoppers in the centre of Glasgow.
One eyewitness described the vehicle knocking people down "like pinballs" as it veered out of control over a distance of around 300 metres, only stopping when it crashed into the side of a hotel.
Others described emergency services battling to save the lives of injured people on the street.
Police in the Scottish city said the the lorry hit several pedestrians as it careered along the road.
The truck, owned by Glasgow City Council, mounted the pavement and hit the first pedestrian outside the Gallery of Modern Art on Queen Street at about 2.30pm, police said.
It struck several more as it continued at speed for another two blocks, crashing at the Millennium Hotel on George Square, near Queen Street station.
The accident happened near Glasgow's main shopping hub, where people filled the streets just three days before Christmas Day.
Witnesses told the BBC people were being treated by ambulance and emergency crews as debris and Christmas shopping was strewn over the street.
The crash was being treated as a road traffic incident and police did not believe there was anything "sinister".
Superintendent Stewart Carle of Police Scotland said: "With all the drama of these people may be alarmed and believed it to be something other than what we believe it to be.
"It is a road traffic collision with multiple fatalities. It is a terrible incident but we don't believe there is anything at this juncture that is sinister about the accident."
One witness told Reuters the truck appeared to have driven out of control along a pavement for up to 70 metres in the city centre shopping area.
Two witnesses on a tour bus are reported as saying the truck driver had been slumped over the steering wheel as it travelled down the street.
The cause of the crash was not immediately clear but there were unconfirmed reports that the driver might have suffered a heart attack. Police said the driver was being treated in hospital .
Prime Minister David Cameron said he was being kept informed of developments, while Scotland's First Minister Nicola Sturgeon tweeted: "My thoughts with everyone involved in dreadful George Square incident".